1. Field of the Invention
The present invention relates to a continuous industrial process for the preparation of zeolites, particularly zeolite A by continuously and instantly mixing a sodium silicate solution and a sodium aluminate solution. The concentration of these solutions and the proportions are carefully controlled. The solutions are mixed at an elevated temperature and the slurry transferred to a crystallization reactor. The product is recovered from the crystallization reactor, washed and dried.
2. Description of the Prior Art
Application of zeolites are based on their well known properties as cation exchangers, described for example in "Comprehensive Treatise on Inorganic and Theoretical Chemistry," by J. W. Mellor, vol. VI, part 2, Longman Editors, 1925, pp 575-579. These zeolites are used in water treatment and in particular for incorporation into the composition of detergents for the sequestration of the calcium in the wash water.
The most widely used of the zeolites is the zeolite designated zeolite A in U.S. Pat. No. 2,882,243 to Milton. This zeolite has the formula Na.sub.2 O, Al.sub.2 O.sub.3, 2 SiO.sub.2, .sup.x H.sub.2 O, where x can vary from 1 to 8 depending on the drying conditions of the product. In the most widely used products x=4 to 5.
To promote the exchange capacities of Ca.sup.++ and the selectivity of the zeolite, the latter should be as pure as possible and consequently be well crystallized, any impurity being either inactive or less selective.
The Na.sup.+ exchange selectivity by Ca.sup.++ is also improved by the use of zeolite grains constituted by agglomeration of crystals so that the diameter of the pores is just slightly larger than the diameter of the Ca.sup.++ ion surrounded by the sphere of the coordinated water molecules.
Finally, when this zeolite is incorporated in a detergent, it is essential that the particle size distribution be restricted to about a median diameter of 2 to 3 microns, sufficiently small to avoid retention of the product in the textile fibers but sufficient to permit an easy solid/liquid separation during fabrication.
Synthesis and the ion exchange properties of synthetic zeolites and in particular zeolite A have been well known for many years (see "Ion Exchange" of Friedrich Helfferich, 1962, McGraw-Hill Book Company, Chapter 2, pages 10-16).
The synthesis processes are all performed in batches by mixing of elements Si, Al and Na brought in as various compounds thus forming a silicoaluminate gel that precipitates. This gel is then crystallized to zeolite A by aging in the mother liquor containing an amount of free soda and soluble alumina compatible with the type of zeolite obtained. Such processes are described, for example, in U.S. Pat. Nos. 2,841,471 and 2,847,280, French Pat. No. 1,404,467 and DAS No. 2,517,219.
These batch precipitation cause considerable variations of the Na.sub.2 O, Al.sub.2 O.sub.3 and SiO.sub.2 contents of the liquor coming from the mixture. This results in a considerable heterogeneity of the resulting product from the viewpoint of particle size, crystallinity and exchange selectivity (size of the pores).
On the other hand, in all the described processes the crystallization mother liquor, still containing not inconsiderable amounts of soda and alumina, is discarded causing a considerable consumption of raw materials and a considerable pollution.
The applicant has developed a continuous process for preparing zeolite A which remedies these difficulties.
This process is a continuous homogeneous precipitation and crystallization process which makes it possible to obtain a homogeneous and constant quality of the zeolite A product.